Photoelectric control of bimetal-type switch with plural heaters



March 11, 1969 YASUTAKA KAWASE 3,432,728

PHOTOELECTRIG CONTROL OF BIMETAL-TYPE SWITCH WITH PLURAL HEATERS FiledFeb. 27, 1967 FIG.

FIG.

FIG.

no 5 (Km DAY RESISTANCE OF PHOTOCONDUCTIVE ELEMENT I NVENTOR YASUTAKAKAWASE y pwla gi M United States Patent 2 Claims ABSTRACT OF THEDISCLOSURE A light sensitive control apparatus having a thermal switchconsisting of a photocell, a pair of electric heating resistors whichare thermally connected to a pair of bimetal strips, one of which beingmain and the other temperature compensating. The apparatus can beoperated to turn on and otf a lamp located outdoors in response toambient light.

The present invention relates to a light sensitive control means, andmore particularly, to an improved photosensitive electric switch whichis in response to ambient light for automatically controlling a circuitconnected to a lamp positioned outdoors.

Prior to the description of the present invention, the conventionallight sensitive switch of the prior art is summarized hereinbelow. Asshown in FIG. 1 of the accompanying drawing, there are shown aphotoconductive or photosensitive cell or photocell 1 for detectingambient light, a heating resistor or heater 2 which is connected with asource of commercial, say 100 v. A.C. supply across the photocell 1, amain bimetal strip 4 which is thermally connected with the heater 2, anda temperature compensating bimetal strip 5 positioned in proximitythereto. A thermal switch 6 consists of both bimetal strips and theheater 2.

In the light sensitive switch of the above construction, the photocell 1exhibits a very low value of resistance during the daylight time, hencethe heater 2 attains a maximum heat value so that the main bimetal strip4 curves outwardly or leftwardly in FIG. 1 as shown by a broken line. Inthis state, the thermal switch 6 maintains an open circuit wherein thereis no light. However, the photocell 1 shows a very high value ofresistance during the night time where an ambient intensity ofillumination is low, hence the heat value of heater 2 becomes minimum.As a result, the main bimetal strip 4 stands erect so that the thermalswitch 6 maintains a closed circuit wherein there is light.

In the conventional photosensitive switch described above, where astreet lighting post is employed as a load to be controlled, it happensthat the street lamp is on while it is still light in the eveningtwilight, and the street lamp is olf while it is already in the daylightafter sunrise. As a result, it follows that a futile consumption ofelectric energy together with a shortened life of an electric bulb wouldbe inevitable. To overcome the above disadvantage has been one of thekeen desires in industry.

The cause of the disadvantage is described as follows: During the darknight, a high voltage equal to a peak value, say, 130-150 v. of thecommercial 100 v. A.C. source applies to the photocell continuously. Tomeet the safety requirement, the photocell is required to have a3,432,728 Patented Mar. 11, 1969 characteristic of withstanding avoltage of 250-300 v. In view of this requirement, the photocell havingsuch a particular characteristic has been in general use.

In reference to the photoconductive or photosensitive element, it isknown that sensitivity varies inversely to the square of peak-voltagerating between peak-voltage rating and sensitivity. The sensitivity ofthe photosensitive element is shown by the relation between illuminationintensity and resistance, and a high sensitivity refers to a lowresistance.

For example, to increase two times as high as the peakvoltage rating ofthe photoconductive element is to increases two times as distant as thespace between comblike electrodes. Hence, the number of comb tooth perunit area of light received decreases one half, so the value of seriesresistance of the photoconductive element increases two times while thevalue of parallel resistance decreases one half. In consequence, itssensitivity will be reduced to one-fourth.

In view of the above, in general, the sensitivity of the photoconductiveelement having the characteristic of resistance to a high voltage, say,250-300 v. is considerably low, and the street lamp turns on in responseto a higher ambient intensity of illumination than desired, and otf to arelatively lower ambient light.

In accordance with a preferred embodiment of the invention, the voltageapplied to the photoconductive element at night is reduced to about onehalf of that of the conventional one, a photoconductive element of arelatively low peak-voltage rating, in other words, of a relatively highsensitivity can be used so that the above disadvantage can be obviated.

Accordingly, it is an object of the invention to provide an improvedlight sensitive control means in which a photoconductive element of alower peak-voltage rating resistance to voltage, in other words, of ahigher sensitivity than before can be used.

It is another object of the invention to provide an improved lightsensitive control or photosensitive switch means for automaticallycontrolling a circuit connected to a lamp in response to ambient lightwherein a less futile consumption of electric energy and a lessshortened life of a bulb than before can be attained.

Further objects and advantages of the present invention will be apparentfrom the following description, reference being had to the accompanyingdrawing, wherein a preferred embodiment of the present invention isshown.

In the drawing:

FIGURE 1 is a diagrammatic view of an electric circuit of a lightsensitive control means of the prior art.

FIG. 2 is a diagrammatic view of an electric circuit of a lightsensitive control means in accordance with the principle of the presentinvention.

FIG. 3 is a diagrammatic side view showing the relation between athermal switch and a heating resistor or heater of the control means ofthis invention.

FIG. 4 is a characteristic diagram showing the current consump;tion oftwo heaters in response to the ambient illumination versus theresistance value of photoconductive element.

In the drawing, it is to be noted that the numerals indicting the partsshown in FIG. 1 are different from those of parts in FIGS 23, though theparts are the same.

A photosensitive switch embodying the principle of the invention will bedescribed hereinbelow in connection with a preferred embodiment thereofshown in FIGS. 2-3.

In FIG. 2, there is shown a circuit comprising a photoconductive element7 consisting of CdS (Cadmium sulfide) or CdSe (Cadmium selenide) fordetecting an ambient light connected in series with a first heatingresistor or heater 8 with a terminal 9 and a terminal 10, a secondheating resistor or heater 11 connected in parallel with thephotoconductive element 7, a thermal switch 12 connected with terminalsand 13, a load or lamp to be controlled connected with terminals 9 and13, and a commercial 100 v. A.C. source connected with terminals 9 and10.

In FIG. 3, the thermal switch 12 comprises a main bimetal strip 16 whichbends leftwardly as shown in a broken line or opens the circuit inresponse to the increase of temperature, and a temperature compensatingbimetal strip 17 positioned in proximity thereto. The temperaturecompensating bimetal strip 17 bends leftwardly or inwardly in responseto the increase of temperature, too. The main bimetal strip 16 isthermally connected with the first heater 8 while the temperaturecompensating bimetal strip 17 is also thermally connected with thesecond heater 11. The term, thermally connected means that two parts areconnected by heat, in other words, when a predetermined heat value isreached, two parts are connected, and when it is lowered, they part fromeach other. The load connected with the terminals 9' and 13 refers to astreet lamp located outdoors, other lamp or equip ment. The source 15connected with the terminals 9 and 10 is the commercial 100 V. AC.

The first heater 8 and the second heater 1.1 are preferably of the sameconfiguration, and it is to be noted that the resistance value of bothheaters are specified to be of 15 kiloohms, respectively. In addition,the main bimetal strip and the temperature compensating bimetal stripare preferred to be of substantially the same material andconfiguration. Both bimetal strips can be formed integral with two legsin the form of H. However, the invention is not limited as above.

In many kinds of the conventional photosensitive switch of the similartype, the heating resistor or heater has been thermally connected withthe main bimetal strip by winding the former around the latter orbonding the former to the latter. Besides, in the conventional device ofthe same type including the main bimetal strip and the temperaturecompensating bimetal strip, it is found that the temperaturecompensation effect obtained by the two bimetal strips is notsatisfactory due to a mechanical resistant action of the main bimetalstrip by the heater when subjected to bending. In consequence, it hasoften led to an erroneous action.

However, in the light sensitive control means of this invention, theheating resistor or heater is thermally connected with both temperaturecompensating bimetal strip and main bimetal strip, hence the abovedisadvantage has been overcome simultaneously.

In the photosensitive switch of this invention during the daylight time,the photoconductive element 7 reduces its resistance value to the orderof about 100 ohms to make the heat value of the first heater 8 to bemaximum while that of the second heater 11 to minimum, almost zero.Accordingly, during the daylight time the main bimetal strip 16 bendsoutwardly or leftwardly in a broken line as shown in FIG. 3 to maintainthe thermal switch 12 to be open.

On the contrary, however, during the dark night period, thephotoconductive element 7 shows a higher resistance value than 1 megaohmto apply about 50 v. on the heaters8 and 11, respectively, hence theheat values of both heaters 8 and 11 are substantially equal so thatboth main bimetal strip 16 and temperature compensating bimetal strip 17bend in the same direction in the same amount to maintain the thermalswitch 12 to be closed.

In FIG. 4 the characteristic diagram showing the current consumption oftwo heaters is shown in which A refers to an operating diagram of heater8 while B to that of heater 11.

It is understood that a known chattering prevention means made of magnetor spring is preferably provided between the main bimetal strip 16 andthe opposite bimetal strip 17, though not shown in the drawing.

In the light sensitive control means of this invention, the voltageapplied to the photoconductive element 7 in the commercial V. AC. sourcenever exceeds about 70 v. even at its peak day and night. Therefore, thephotoconductive element 7 having the characteristic of resistance tovoltage as small as half of that of the prior art device, say, in theorder of v. can be employed, hence the sensitivity thereof can bedeveloped to four times as high as that of prior art in comparison withthe conventional device.

In the conventional device of FIG. 1, in order to effect an assureddesign for operation of thermal switch 6 which is in response to thevariation of illumination of photocell 1, it is known that the centralpoint of operation of thermal switch should be determined as a pointwhere the resistance value of photocell 1 is equal to that of heater 2.Accordingly, when the resistance value of photocell 1 is 15 kiloohms, inother words, when the heater 2 evolves the heat value of 166 mw.(suppose the source voltage 100 v.), it is specified as the centralpoint of operation of thermal switch 6.

However, in'the device of this invention, when the heat difference of166 mw. takes place between two heaters 8 and 11, the resistance valueof photocell 7 is 22.5 kiloohms as clearly shown in FIG. 4. Therefore,in the device of this invention, the central point of operation ofthermal switch can be shifted to the side of low intensity ofillumination compared with the conventional one, in other words, 1.5times (22.5 kiloohms divided by 15 kiloohms) as high as before. Thus,the sensitivity of the device of this invention can be made 1.5 times ashigh as ever. As described above, the device of this invention candevelop its sensitivity about four times as high as prior art, andfinally improve its sensitivity to such a considerable degree as aboutsix times as high as ever when put together.

To put it concretely, assuming that the conventional device can operateto turn on lamp at the illumination of 60 luxes, and off at luxes, ithas been found that the device of the present invention can operate toturn on lamp at 10 luxes and off at 30 luxes.

Furthermore, as the heater 11 is thermally connected with the mainbimetal strip 16 as well as the temperature compensating bimetal strip17, the mechanical resistance of heater against the bending action ofbimetal acts on the main bimetal strip 16 as well as the temperaturecompensating bimetal strip 17 in a uniform manner. As a result, asatisfactory effect of temperature compensation can 'be achieved.

As fully described in the foregoing, the light sensitive control meansof this invention can improve its sensitivity considerably by a simplereconstruction comprising adding a single heating resistor or heater tothe conventional device of the same type. In addition, an improvedadvantage is obtained from the device of this invention in order toutilize the temperature compensation effect efficiently even when theheater winds around the bimetal strip or attaches to it.

While the embodiment of the invention as herein disclosed constitutes apreferred form, it is to be understood that other forms might beadopted.

What is claimed is as follows:

1. In a light sensitive control comprising a photoconductive elementadapted to detect an ambient illumination, a first heater connected inseries with said element, a thermal switch comprising a main bimetalstrip tending to bend outwardly in response to the increase of itstemperat-ure, said main bimetal strip having a first contact, atemperature compensating bimetal strip tending to bend inwardly inresponse to the increase of its temperature, said temperaturecompensating bimetal strip having a second contact opposing said firstcontact, and said heater being thermally connected with said two bimetalstrips, the improvement comprising a second heater connected in parallelwith said element, and said second heater being also thermally connectedwith said temperature compensating bimetal strip whereby said contacts10 open and close in response to light and the absence of light on saidphotoconductive element.

2. In a light sensitive control means as claimed in claim 1 wherein saidphotoconductive element is provided with a pair of comb-like electrodespositioned face-to-face.

References Cited UNITED STATES PATENTS 2,654,865 10/1953 Klug 324-1062,796,553 6/1957 Ronning 250206 X 2,967,981 1/1961 Wise 317-124 JAMES W.LAWRENCE, Primary Examiner. E. R. LA ROCHE, Assistant Examiner.

US. Cl. X.R. 250206; 307-117

